This error occurs when you open an article or type text but a font is incorrectly loaded in GN4. Most possibly it was imported with a inappropriate font layout which does not cover all necessary characters.

Make sure you use UNICODE font layout for all letter fonts, and SYMBOL or DINGBATS font layout for other fonts.

To correct this error

Reimport the fonts with the appropriate font layout.

Restart GN4 application and retry.